Method and apparatus for conveying a cellulosic feedstock

ABSTRACT

A method and apparatus for conveying a cellulosic feedstock are disclosed. The apparatus, such as a holding tank, has a passage. At the outlet to the passage, a conveyance member is provided to convey the cellulosic feedstock laterally across the outlet. The conveyance member, e.g., at least one screw conveyor, may have a variable pitch along its length. The method comprises passing the cellulosic feedstock through an impregnation chamber to an outlet of the impregnation chamber; passing the cellulosic feedstock from the outlet of the impregnation chamber to an inlet of a holding tank; passing the cellulosic feedstock downwardly through the holding tank; maintaining a generally constant residence time in the holding tank; and subsequently subjecting the cellulosic feedstock to a hydrolysis process.

FIELD

The invention relates to a method and apparatus for obtaining andconveying a cellulosic feedstock, which may be used for the subsequentproduction of a fermentable sugar stream from the cellulose andhemicellulose in the cellulosic feedstock wherein the fermentable sugarstream may be used for subsequent ethanol production. More specifically,the invention relates to a method and apparatus for withdrawing one ormore feedstock stream from a holding tank.

BACKGROUND

Several processes for the production of ethanol are known. Generally,the production of fuel ethanol involves the fermentation of sugars withyeast. Typically, the sugars are derived from grains, such as corn andwheat. The starches in the grains are subjected to enzymatic hydrolysisin order to produce the sugars, which are then subjected to fermentationto produce ethanol.

Plant materials are a significant source of fermentable sugars, such asglucose that can be transformed into biofuels. However, the sugars inplant materials are contained in long polymeric chains of cellulose andhemicellulose. Utilizing current fermentation processes, it is necessaryto break down these polymeric chains into monomeric sugars, prior to thefermenting step.

Recently, processes have been developed for utilizing cellulosicfeedstock, such as corncobs, straw, and sawdust, to produce sugars forethanol fermentation. Such processes typically comprise pre-treating thefeedstock to increase the accessibility of the cellulose to hydrolysisenzymes, and subjecting the cellulose to cellulase enzyme systems toconvert the cellulose into glucose.

Methods of converting plant biomass into fermentable sugars are known inthe art and in general comprise two main steps; a pre-treatment step toactivate the plant structure, and an enzymatic or chemical hydrolysisstep to convert the polymeric chains of cellulose and hemicellulose intomonomeric sugars. Several approaches have been used for thepre-treatment step, e.g., autohydrolysis, acid hydrolysis, ammoniaactivation, kraft pulping, organic solvent pulping, hot waterpre-treatment, ammonia percolation, lime pre-treatment, caustic sodapulping, or alkali peroxide pre-treatment. Early pre-treatment stepsincluded grinding or milling the feedstock into a powder, which was thenmixed with water to form a slurry.

More recently, solvent based pre-treatments, alkali pre-treatments, andacidic pre-treatments have also been described. PCT publicationWO/2007/009463 to Holm Christensen describes an alternate pre-treatment,which does not involve the addition of acids, bases, or other chemicals.This pre-treatment process involves soaking the cellulosic material inwater, conveying the cellulosic material through a heated andpressurized reactor, and pressing the cellulosic material to produce afiber fraction and a liquid fraction. After pressing the cellulosicmaterial, the cellulosic material is exposed to hydrolysis enzymes.

Each pre-treatment technology has a different mechanism of action on theplant structure, inducing either physical and/or chemical modifications.However, the main objective of the pre-treatment is to provideaccessibility of the plant material to the enzymes.

SUMMARY

The commercial viability of a hydrolysis process is dependent on thecharacter of the feedstock provided to the hydrolysis unit. Typically,this requires that a feedstock is activated such that a significantportion (e.g., greater than 75%) of the cellulose and hemicellulose ofthe feedstock is accessible to hydrolysis enzymes. If such an activatedfeedstock is provided to an enzymatic hydrolysis unit, then at least60%, preferably more than 75% and more preferably over 90% of thecellulose and hemicelluloses may be converted to monomeric sugars. Thissugar rich process stream may subsequently be subjected to fermentationto produce an alcohol stream. The alcohol stream from the fermentationstage (i.e., the raw alcohol stream) may have an ethanol content ofabout 3-22% v/v, preferably about 5-15% and more preferably more about8-12%.

An activated feedstock for enzymatic hydrolysis is preferably preparedby autohydrolysis, which is preferably conducted in a steam explosionreactor also known as a hydrolyzer (also known as a digester).Autohydrolysis is a process of breaking down hemicellulose and celluloseby exposure to high temperatures, steam and pressure, sometimes in thepresence of an added chemical agent, such as an organic or inorganicacid, e.g., sulphuric acid. When performed in the presence of an addedacid, the reaction is known as an acid hydrolysis.

During autohydrolysis, the degree of polymerization of cellulose andhemicellulose may be reduced from about 10,000 to about 1,500-1,000.This process is preferably carried out above the glass transitiontemperature of lignin (120-160° C.). Depending upon the severity of thereaction, degradation products may be produced, such as furfural,hydroxyl-methylfurfural, formic acid, levulinic acid and other organiccompounds.

During a steam explosion treatment (more commonly called autohydrolysisif no externally added catalyst), a cellulosic feedstock is subjected toelevated heat (e.g., 180° C. to 220° C.) and pressure (e.g., 130 psig to322 psig) optionally in the presence of suitable chemicals (e.g.,organic/and/or inorganic acids, ammonia, caustic soda, sulfur dioxide,solvents etc.) in a pressurized vessel. Preferably, external chemicaladdition is not utilized; in which case, the only catalyst that may bepresent may be acetic acid that is generated in situ. The treatedcellulosic feedstock is then released from the pressurized vessel suchthat the pressure is rapidly reduced (e.g., 1 second or less). Thebiomass may exit the hydrolyzer into a reduced pressure, preferablyatmospheric pressure and, more preferably into a vacuum. The rapiddecrease in pressure results in the biomass separating into individualfibers or bundles of fiber. This step opens the fiber structure andincreases the surface area. The lignin remains in the fiber along withcellulose and residual hemicellulose. Accordingly, the explosive releaseof pressure, combined with the high temperature and pressure treatmentresults in the physicochemical modification of the cellulosic feedstockthat is then suitable for feeding to an enzymatic hydrolysis unit.

In order for the steam explosion process to be able to produce anactivated feedstock that is capable of producing such a sugar richprocess stream, the temperature and moisture level of the cellulosicfeedstock that is fed to a steam explosion reactor preferably isrelatively uniform and preferably has a temperature from about 50 toabout 70° C., and more preferably 50-65° C. and a moisture content fromabout 30 to about 60 wt % (preferably 45 to about 55 wt %). Moisturecontent is the quantity of water contained in a material, and on aweight basis, is the weight of water in the material divided by the massof the material.

Embodiments of the present invention provide a method and apparatus forwithdrawing a cellulosic feedstock from a vessel by actively withdrawingfeedstock from different portions of the outlet of a vessel and,preferably, withdrawing a feedstock evenly from across the outlet. Anadvantage of this design is that a generally uniform residence time ofthe feedstock in the vessel may be achieved. For example, the varianceof the residence time may be up to 5 minutes, preferably, less than 3minutes and more preferably less than 2 minutes. Accordingly, thetendency of the portion of a feedstock that is adjacent a heated surfacein the vessel, such as a surface in thermal contact with a heatingjacket, to be degraded by overheating may be reduced and, preferably,eliminated.

Alternately, or in addition, embodiments of the present inventionprovide a method and apparatus for withdrawing a cellulosic feedstockfrom a vessel in a direction transverse to the direction of travel ofthe material through the vessel. Accordingly, if the vessel is orientedsuch that they passage therethrough is generally vertical, the feedstockis withdrawn generally horizontally. A plurality of feedstock streams,each of which may be conveyed in a different direction may be obtained.

According to a first broad aspect, a holding tank apparatus forpreparing a cellulosic feedstock is provided. The holding tank apparatuscomprises at least one sidewall defining a volume having an upperportion and a lower portion. At least one inlet is provided adjacent theupper portion, and at least one outlet is provided adjacent the lowerportion, at an elevation below the inlet. At least one screw conveyor isprovided for conveying the cellulosic feedstock laterally across theoutlet. The at least one screw conveyor has a variable pitch along itslength. Providing the at least one screw conveyor with a variable pitchalong its length may allow for a generally equal amount of cellulosicfeedstock to be withdrawn from all portions of the outlet.

In some embodiments, the at least one screw conveyor has a first end anda second end, and the pitch at the first end differs from the pitch atthe second end. In some further embodiments, the at least one screwconveyor comprises a first region having a first range of pitch, and asecond region having a second range of pitch. In yet furtherembodiments, the at least one screw conveyor comprises an intermediateregion between the first region and the second region, and theintermediate region has a third range of pitch between the first rangeof pitch and the second range of pitch.

In some embodiments, the at least one screw conveyor has a first end anda second end, and the pitch decreases from the first end to the secondend and, preferably, varies at a constant rate between the first end andthe second end.

In some embodiments, the outlet defines a plurality of portions and thepitch varies such that approximate equal portions of feedstock arewithdrawn from each portion of the outlet.

In some embodiments, the at least one screw conveyor comprises aplurality of screw conveyors and at least one of the screw conveyorsconveys the cellulosic feedstock in a first direction and at least oneof the screw conveyors conveys the cellulosic feedstock in a seconddirection. In a further embodiment, the first direction and the seconddirection are substantially opposite.

In some embodiments, the at least one screw conveyor comprises aplurality of screw conveyors and at least two adjacent two screwconveyors have a length and convey the cellulosic feedstock in a firstdirection and the pitch of the screw conveyors at any location along thelength of the screw conveyors is essentially the same.

In some embodiments, the apparatus comprises a plurality of screwconveyors having a length, and a first pair of the screw conveyorsconveys cellulosic feedstock in the first direction, and a second pairof the screw conveyors conveys cellulosic feedstock in the seconddirection.

In some such embodiments, the pitch of the first pair of screw conveyorsat any location along the length is essentially the same and the pitchof the second pair of screw conveyors at any location along the lengthis essentially the same. In some further embodiments, the first pair ofscrew conveyors conveys cellulosic feedstock in the first direction, andthe second pair of screw conveyors conveys the cellulosic feedstock in asecond direction opposite to the first direct, and the pitch of thefirst pair of screw conveyors is a mirror image of the pitch of thesecond pair of the screw conveyors.

In some embodiments, the first pair of screw conveyors is rotatable in afirst direction, and the second pair of screw conveyors is rotatable ina second direction opposite to the first direction.

In some embodiments the at least one screw conveyor extends across allof the outlet.

In some embodiments, the apparatus further comprises a heating jacketprovided on at least a portion of the apparatus. Such embodiments may beadvantageous because the heating jacket may elevate or maintain thetemperature of the cellulosic feedstock, which may render the cellulosicfeedstock further accessible to auto hydrolysis, preferably followed byenzymes hydrolysis.

In some embodiments, the lower portion of the holding tank has a greatercross sectional area then the upper portion of the holding tank. Suchembodiments may be advantageous because cellulosic material may beprevented from adhering or sticking to the sidewalls as the cellulosicmaterial passes through the holding tank. Accordingly, each portion ofcellulosic feedstock that passes through the holding tank may haveessentially the same residence time in the volume.

In some embodiments, the sidewalls comprise a first sidewall and asecond sidewall opposed to the first sidewall, and the first and secondsidewalls diverge relative to each other from the upper portion to thelower portion.

In some embodiments, the sidewalls comprise a third sidewall and afourth sidewall opposed to the third sidewall, and the third and fourthsidewalls extend between the first and second sidewalls, and the thirdand fourth sidewalls diverge relative to each other from the upperportion to the lower portion.

In another broad aspect, a method of preparing a cellulosic feedstockfor ethanol production is provided. The method comprises passing thecellulosic feedstock through an impregnation chamber to an outlet of theimpregnation chamber; passing the cellulosic feedstock from the outletof the impregnation chamber to an inlet of a holding tank, the holdingtank having an outlet; passing the cellulosic feedstock downwardlythrough the holding tank; maintaining a generally constant residencetime in the holding tank; and subsequently subjecting the cellulosicfeedstock to a downstream hydrolysis process.

In some embodiments, the method further comprises maintaining atemperature in the holding tank between about 50° Celsius and about 75°Celsius.

In some embodiments, the method further comprises maintaining a varianceof the residence time of each portion of the amount of up to 5 minutes.

In some embodiments, the method further comprises maintaining aresidence time in the holding tank from about 10 to about 45 minutes.

In some embodiments, the method further comprises conveying thecellulosic feedstock laterally across an outlet of the holding tank. Insome such embodiments, the cellulosic feedstock is actively withdrawnfrom essentially the entirety of the outlet. Further, in some suchembodiments a generally equal amount of cellulosic feedstock iswithdrawn from each portion of the outlet. Further, in some suchembodiments, the cellulosic feedstock is conveyed in at least twodifferent lateral directions.

In some embodiments, a screw conveyor having different pitches isutilized to convey the cellulosic feedstock laterally across the outletof the holding tank.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages of the present invention will be more fullyand particularly understood in connection with the following descriptionof the preferred embodiments of the invention in which:

FIG. 1 is a perspective illustration of an embodiment of a holding tankof the present invention, showing an impregnation chamber positionedupstream from the holding tank;

FIG. 2 is a perspective illustration of the impregnation chamber of FIG.1;

FIG. 3 is a top cutaway view of the impregnation chamber of FIG. 1;

FIG. 4 is a cross section taken along line 44 in FIG. 2;

FIG. 5 is a cross section taken along line 5-5 in FIG. 1;

FIG. 6 is a perspective view of an embodiment of housing of the presentinvention, shown removed from a holding tank;

FIG. 7 is a cross-section taken along line 7-7 in FIG. 6;

FIG. 8 is a top view of the housing of FIG. 6;

FIG. 9 is a top view of an alternate embodiment of a housing of thepresent invention; and,

FIG. 10 is a cross-section taken along line 10-10 in FIG. 1.

DETAILED DESCRIPTION

Embodiments of the present invention provide enhanced uniformity ofresidence time of the feedstock in a holding tank. In particular,despite withdrawing the feedstock laterally with respect to thedirection of flow through the holding tank, a generally uniformresidence time for all portions of the feedstock that are concurrentlywithdrawn from the holding tank may be obtained.

An embodiment of an apparatus of the present invention is shown in FIGS.1-10. It will be appreciated that although the method is described withreference to the apparatus and vice versa, the method may be carried outwith an alternate apparatus, and the apparatus may be used according toan alternate method. Furthermore, although the method is described as acontinuous process, it will be appreciated that the method may becarried out as a semi-continuous or batch process.

The cellulosic feedstock is preferably a lignocellulosic feedstock. Alignocellulosic feedstock is derived from plant materials. As usedherein, a “lignocellulosic feedstock” refers to plant fiber containingcellulose, hemicellulose and lignin. In some embodiments, the feedstockmay be derived from trees, preferably deciduous trees such as poplar(e.g., wood chips). Alternately or in addition, the feedstock may alsobe derived from agricultural residues such as, but not limited to, cornstover, wheat straw, barley straw, rice straw, switchgrass, sorghum,bagasse, rice hulls and/or corn cobs. Preferably, the lignocellulosicfeedstock comprises agricultural residues and wood biomass, morepreferably wood biomass and most preferably deciduous. The applicantscontemplate other sources of plant materials comprising cellulose,hemicellulose and/or lignin, such as algae, for use in derivingcellulosic feedstocks and any of those may be used. The lignocellulosicfeedstock is preferably cleaned, e.g., to remove ash, silica, metalstrapping (e.g., from agricultural products), stones and dirt. The sizeof the components of the lignocellulosic feedstock may also be reduced.The size of the components of the feedstock may be from about 0.05 toabout 2 inches, preferably from about 0.1 to about 1 inch, and morepreferably from about 0.125 to about 0.5 inches in length. For example,the cellulosic feedstock may comprise fibers, e.g., chopped straw, of alength of between about 0.16 inches and about 0.28 inches. Any processmachinery that is able to crush, grind or otherwise decrease theparticle size may be utilized.

The feedstock is preferably treated with water so as to have a moisturecontent upon entry to holding tank 100 of between about 30 and about 60preferably between about 45 and about 55 wt %. For example, referring toFIGS. 1 and 2, an embodiment of a holding tank apparatus 100 of thepresent invention is shown wherein the holding tank 100 is positioneddownstream from a water impregnation reactor such as impregnationchamber 10, which is preferably used to pre-treat the feedstock prior tothe feedstock entering holding tank 100. Impregnation chamber 10 ispreferably configured to pre-treat the cellulosic feedstock, for exampleby moistening and/or heating the cellulosic feedstock.

A preferred impregnator 10 is exemplified in FIGS. 2-4. As showntherein, in some embodiments, an impregnator feeder 30, namely a feederthat conveys feedstock into impregnation chamber 12, is preferablypositioned upstream of mixing or impregnation chamber 12. Feeder 30 maybe of any design. Preferably, feeder 30 is of a design that inhibits,and preferably prevents, the flow of moisture upstream of feeder 30. Forexample, a rotating valve or the like may be provided to segment suchupstream flow. Preferably impregnation feeder is a screw feedercomprising a motor 32 drivingly connected to a screw or auger 34positioned below an inlet, such as via a transmission or gear reductionassembly provided in housing 36. The shaft on which screw 34 is providedmay be rotatably mounted in housing 38 such that augur 34 is acantilevered plug screw conveyor. Accordingly, feeder 30 produces a plugof material that prevents upstream migration of moisture. The plug maybe conveyed into inlet housing 40 that is mounted to impregnationchamber 12. The feedstock may then pass downwardly into impregnationchamber 12.

Impregnator 10 may comprise an inlet 42 positioned below inlet housing40, one or more conveyance members 14 for urging the cellulosicfeedstock along the length of chamber 12, one or more moisture injectionports 16, which may be provided on paddles 20 of conveyance member 14and/or inner wall 22 of impregnator 10, for injecting moisture into thecellulosic feedstock one or more heating jackets 18 provided outward ofinner wall 22 for heating the cellulosic feedstock, and an outlet 24.Heating jacket 18 may comprise an outer wall 26 spaced from inner wall22 to define a passage 28 through which a heated fluid, e.g. water, maypass.

As exemplified in FIG. 2, one or more conduits 54 may convey water to aplurality of branch conduits 56 extending to different locations on theupper portion of chamber 12. The end of these conduits are in fluid flowcommunication with the interior of chamber 12, via, e.g., a moistureaddition member such as a nozzle or an open ended pipe or the like.

As exemplified, conveyance members 14 are rotatably mounted in chamber12 and are drivenly connected to a motor 46. As exemplified, motor 46 isdrivingly connected to conveyance members 14 via a transmission or gearreduction assembly provided in housing 48. The gear reduction assemblymay be drivingly connected to ends 50 of conveyance members 14 that arepositioned inside housing 52.

In order to prevent material stagnating in impregnator 10, impregnator10 may have a bottom wall 44 that has two or more portions each of whichhas a conveyance member 1 associated therewith. Bottom wall 44 andconveyance member 14 are preferably configured such that bottom wall 44is swept as conveyance member 14 rotates. For example, as exemplified inFIG. 4, bottom wall 24 may be scallop shaped, e.g., have two invertedarches or troughs. Further details regarding various embodiments ofoptional impregnation chamber 10 may be found in co-pending U.S. patentapplication Ser. No. 12/181,596, filed on Jul. 29, 2008, the disclosureof which is incorporated herein by reference in its entirety. Inalternate embodiments, impregnation chamber 10 may pre-treat thecellulosic feedstock in another manner, and the invention is not limitedin this regard.

After the cellulosic feedstock is optionally pre-treated in impregnationchamber 10, it is directed to holding tank apparatus 100, e.g., viaoutlet passage 58 that is downstream from outlet 24 of chamber 12, whereit is held or contained for a residence time, such that for example,moisture added in impregnation chamber 10 has sufficient time topenetrate into the feedstock so that the feedstock is ready fordownstream processing. Alternately, or in addition, the feedstock mayrequire additional time for all portions of the feedstock to be raisedto a predetermined temperature that is suitable for downstreamprocessing. Alternately, the feedstock entering holding tank 100 may beat the predetermined conditions for downstream processing and holdingtank is used as a reservoir to hold prepared feedstock such thatdownstream processes may operate on a continuous basis. From holdingtank 100, the cellulosic feedstock may be directed to one or morehydrolysis reactors, preferably one or more autohydrolysis reactorsfollowed by one or more enzymatic hydrolysis reactors (not shown)positioned downstream from the holding tank apparatus 100 such that thecellulose may be hydrolyzed to produce sugars that are suitable forfermentation to ethanol.

As exemplified in FIGS. 1 and 5, holding tank 100 is oriented such thatthe passage through holding tank 100 preferably extends generallydownwardly and the passage therethrough is preferably configured so asto reduce, and more preferably essentially prevent, bridging offeedstock in holding tank 100. Further, the passage from impregnator 10to holding tank 100 preferably extends generally downwardly.Accordingly, it is preferred that the passage through holding tank 100extends generally downwardly and that the passage has a greater crosssectional area at the lower end then the upper end. More preferably, thecross sectional area continually increases in the downward direction.This may be achieved by constructing the passage of the holding tankwith one or more walls that diverge in the downward direction.

If the feedstock passing downwardly through holding tank 100 interlocks,it may form a blockage by a process known as bridging. The blockage mayextend all the way across the passage in holding tank 100 therebypreventing downward movement of feedstock and causing a gap in thesupply of feedstock to the downstream process unit. Alternately, it mayblock only part of the passage. In any event, intervention would then berequired to remove the blockage. The interruption of feedstock deliveryto the downstream process unit could require part of a plant to be shutdown while the blockage is removed thereby reducing throughput and alsorequiring the plant to be brought back to steady state operatingconditions once the blockage is cleared. Accordingly, the holding tankmay require monitoring to permit intervention at an early stage shouldbridging occur. By increasing the cross sectional area in the downstreamdirection, the tendency of the feedstock to form a blockage of thepassage is reduced and may be eliminated.

As exemplified in FIGS. 5 and 10, holding tank 100 comprises at leastone sidewall 102, which defines a volume or passage 104. In theembodiment shown, holding tank apparatus 100 comprises four sidewalls,namely front wall 102 a, and a spaced apart opposed rear wall 102 b, anda side wall 102 c, and a spaced apart opposed side wall 102 d, andfurther comprises a top wall 103. Accordingly, passage 104, which isdefined by sidewalls 102 a, 102 b, 102 c and 102 d, is rectangular intransverse section. In other embodiments, holding tank apparatus 100 maycomprise, for example, a single rounded sidewall so as to have atransverse section that is circular, elliptical or the like. It will beappreciated that any other transverse section may be utilized.

Passage 104 is preferably longitudinally extending, for example alongaxis 105, and comprises an upper portion 106, and a lower portion 108.Passage 104 preferably extends vertically. However passage may extendgenerally vertically (i.e., at an angle to the vertical such thatfeedstock will flow downwardly therethrough under the force of gravity).In some embodiments, passage 104 may have a length along axis 105 ofbetween about 5 ft and about 20 ft.

An inlet 110 is provided adjacent upper portion 106, and an outlet 112is provided adjacent lower portion 108, at an elevation below the inlet110. In the embodiment shown, inlet 110 is defined by an opening in topwall 103, and outlet 112 is defined by the lower ends 114 of sidewalls102. It will be appreciated that inlet 110 may comprise the entirety ofthe top end of holding tank 100 and accordingly, a top wall 103 may notbe required. It will be appreciated that in the preferred embodiment, nolower surface is provided for passage 104 and that the lower end ofpassage 104 is open. Accordingly, feedstock may flow downwardly throughpassage 104 unimpeded until it encounters feedstock stored in holdingtank 100 or until it encounters housing 116. As exemplified, inlet 110is in fluid communication with and receives cellulosic feedstock fromoutlet 24 of impregnation chamber 10 (e.g. it is downstream of outletconduit 58), and outlet 112 is preferably in fluid communication withand directs cellulosic feedstock to one or more autohydrolysis reactors(not shown).

Referring still to FIG. 5, in the preferred embodiment, lower end 108 ofpassage 104 has a greater cross sectional area than upper end 106 ofpassage 104. That is, a transverse cross section taken through passage104 adjacent outlet 112 has a greater cross sectional area than atransverse section taken through passage 104 adjacent inlet 110. Forexample, the cross sectional area taken adjacent outlet 112 may have anarea of between about 40 ft² and about 60 ft², and the cross sectionalarea taken adjacent inlet 110 may have an area of between about 20 ft²and about 40 ft².

Sidewalls 102 may be configured in a variety of ways in order to providelower end 108 with a greater cross sectional area than upper end 106. Inthe embodiment shown, sidewall 102 a and sidewall 102 b are opposed toeach other, and sidewall 102 c and sidewall 102 d are opposed to eachother, and each of the sidewalls diverge from axis 105 going from inlet110 to outlet 112. Accordingly, passage 104 is substantiallyfrusto-pyramidal, and lower end 108 has a greater cross sectional areathan upper end 106. In an alternate embodiment, sidewalls 102 a and 102b may extend substantially parallel to axis 105, and sidewalls 102 c and102 d may diverge from axis 105. In yet another alternate embodiment,holding tank apparatus 100 may comprise a single rounded sidewalldefining a frustoconical passage 104. In yet another embodiment,sidewalls 102 may be stepped. It is preferred that sidewalls 102continually diverge and that they continually diverge for the entirelength of passage 104 as exemplified. Preferably, they diverge at anangle A from the vertical from about 1° to about 20°, preferably fromabout 2° to about 5°. It will also be appreciated that inner surface 138of sidewalls 102 are preferably smooth and clear of projections thatcould be a source causing bridging to occur.

Providing lower portion 108 with a greater cross sectional area thanupper portion 106 may aid in preventing cellulosic material fromadhering or sticking to sidewalls 102 as the cellulosic material passesthrough holding tank apparatus 100. Accordingly, each portion ofcellulosic feedstock that passes through holding tank 100 may haveessentially the same residence time in passage 104.

In alternate embodiments, lower portion 108 of volume 104 may not have agreater cross sectional area than upper portion 106 of volume 104. Forexample, each of sidewalls 102 may extend essentially vertically andparallel to each other.

In some embodiments, the feedstock may travel directly downwardly to thenext process unit, e.g. a steam explosion reactor. In such a case, it ispreferred the passage continually increase in cross sectional area (asopposed to using a hopper). However, it is preferred that the feedstock,after traveling downwardly through passage 104, is conveyed laterally(transverse to axis 105). Further, it is preferred that the feedstock isactively withdrawn from holding tank 104 instead of permitting thefeedstock to passively exit therefrom. Accordingly holding tank 100 mayfurther comprise or be provided with at least one conveyor adjacentoutlet 112 that is configured to actively convey the cellulosicfeedstock laterally across outlet 112 to withdraw the cellulosicfeedstock from passage 104. Referring to FIGS. 5 to 8, in the embodimentshown, the at least one conveyor comprises a plurality of screwconveyors 126, which are housed in a housing 116. The conveyor may beany transport mechanism known in the art to actively transport feedstocklaterally from outlet 112. For example, the conveyor may comprise anauger, a screw conveyor, tabbed flight screw with bars, or the like thatextends transversely to axis 105.

In the embodiment shown, housing 116 comprises a base 118, sidewalls120, and an open top 122. Open top 122 is preferably at least as largeas outlet 112, and is in vertical registration with outlet 112, suchthat material passing through outlet 112 may pass directly downwardlythrough open top 122. It will be appreciated that in alternateembodiments, sidewalls 102 of passage 104 may provide the sidewalls ofhousing 116. That is, sidewalls 102 may extend beyond outlet 112.Accordingly, in such an embodiment, outlet 112 of passage 104 may not bedefined by ends 114 of sidewalls 102, and rather, may be defined by aportion of sidewalls 102 above ends 114.

Housing 116 comprises at least a first housing outlet 124, through whichcellulosic feedstock conveyed by screw conveyors 126 exits housing 116.Cellulosic feedstock exiting housing outlet(s) 124 may pass into one ormore conduits 125, which may, for example, lead to one or more, e.g.,autohydrolysis reactors (not shown). Preferably each conduit 125 isprovided with one or more screw conveyors or the like extending in thedirection of conduit 125. Preferably, more than one outlet 124 isprovided. An advantage of having more then one outlet is that twotreated feedstock streams may be provided from holding tank 100, each ofwhich may be fed to a different downstream process vessel, e.g. adifferent steam explosion reactor.

As exemplified, housing 116 comprises two housing outlets 124 a, 124 bdefined in base 118 (see FIG. 7). Preferably, each outlet 124 ispositioned such that it is not underneath passage 104 (laterally spacedfrom passage 104) and preferably more then one outlet 124 is provided.An advantage of positioning outlets 124 laterally from passage 104 isthat feedstock may be withdrawn from all of outlet 112 and, morepreferably, evenly from across outlet 112. Further, housing outlets 124a and 124 b are preferably positioned on opposite sides of housing 116.Accordingly, housing outlets 124 a and 124 b may direct cellulosicmaterial to two different, e.g., autohydrolysis reactors, positioned onopposite sides of holding tank 100. As exemplified in FIGS. 1 and 6,housing 116 may have upper wall 166 that extends over the portion ofhousing 116 positioned laterally of holding tank 100. Top wall 166 maycover the portion of screw conveyor 126 positioned laterally of holdingtank 100. Optionally, a grate 168, or other member that provides awindow, may be position in top wall 166 above outlet 124. Grate 168permits a worker to observe the travel of feedstock into conduits 125.

As exemplified, the screw conveyors 126 are mounted above base 118, andeach screw conveyor extends transversely to axis 105 across all ofoutlet 112 (i.e. the length L of each screw conveyor extends at leastfrom a first side of outlet 112 to a second side of outlet 112). Eachscrew conveyor 126 comprises a shaft 128 and at least one helical flight130 extending about the shaft, and is configured to rotate to engagematerial exiting outlet 112, and to convey it towards one of the housingoutlets 124. Shaft 128 may be rotatably mounted by any means known inthe art. As exemplified, shaft 128 has one end journalled in a bearinghousing 164 and a second end journalled in a transmission housing 162.

In the embodiment shown, housing 116 comprises four screw conveyors 126,which are arranged in pairs. Each pair comprises two adjacent screwconveyors 126 which convey the cellulosic feedstock in the samedirection towards a common housing outlet 124. In the embodiment shown,first pair 132 a comprises screw conveyors 126 a and 126 b, which rotateabout respective first 134 a and second 134 b generally parallel axes,and second pair 132 b comprises screw conveyors 126 c, and 126 d, whichrotate about respective first 134 c and second 134 d generally parallelaxes. Each of axes 134 are preferably horizontal, but may be at an angleof up to 45° or greater from the horizontal. Accordingly, screwconveyors 126 a and 126 b transport treated feedstock to outlet 124 aand screw conveyors 126 c and 126 d transport treated feedstock tooutlet 124 b, which is on an opposed side to outlet 124 a. It will beappreciated that screw conveyors 126 a, 126 b, 126 c and 126 d extendunder essentially all of outlet 112. Therefore, the screw conveyors 126preferably withdraw treated feedstock for all portions of outlet 112.Alternately, or in addition, each outlet 124 may have one or more screwconveyors 126 or other transport member associated therewith.

Referring still to FIG. 7, as exemplified, screw conveyors 126 a and 126b of first pair 132 a may each be rotated in a direction indicated byarrow A1, to feed material from above in a direction indicated by arrowA2 towards housing outlet 124 a. Further, screw conveyors 126 c and 126d of second pair 132 b may each be rotated in a direction indicated byarrow A3, to feed material from above in a direction indicated by arrowA4 towards housing outlet 124 b.

In order to permit each screw conveyors 126 to be rotated, in aparticular direction of rotation, each screw conveyor may be driven byits own drive motor 160. As shown in FIGS. 6 and 7, each shaft 128extends outwardly past sidewall 120 into a transmission housing 162wherein motor 160 is drivingly connected to shaft 128. Any drivinglinkage known in the art may be used. It will be appreciated that in analternate embodiment, two or more shafts may be driven by a single motor160.

Accordingly, as exemplified, housing outlets 124 a and 124 b arepositioned on laterally opposite sides of housing 116, and each helicalflight 130 is right-handed. Accordingly, direction A1 and direction A3are opposite to each other, and directions A2 and A4 are opposite toeach other. However, in alternate embodiments, housing outlets 124 a and124 b may be positioned on the same lateral side as each other. In suchan embodiment, directions A1 and A3 may be substantially the same, anddirections A2 and A4 may be substantially the same. In yet furtheralternate embodiments, the helical flight 130 of the first pair 132 a ofscrew conveyors 126 a, 126 b, may be right handed, and the helicalflight 130 of the second pair 132 b of screw conveyors 126 c, 126 d maybe left handed. Accordingly, in such an embodiment, directions A1 and A3may be the same, and direction A2 and A4 may be opposite. It will beappreciated that each pair of screw conveyors 126 may be configured suchthat they rotate in opposite directions. For example, screw conveyor 126a may be configured to rotate clockwise and screw conveyor 126 b may beconfigured to rotate counterclockwise.

It will be appreciated that in alternate embodiments, one or more screwconveyors 126 may be otherwise configured. For example, housing 116 maycomprise only one screw conveyor 126 and one outlet 124, or housing 116may comprise a plurality of screw conveyors which are not arranged inpairs (e.g. the screw conveyors may arranged in sets of three, or assingle screw conveyors), or housing 116 may comprise more than two pairsof screw conveyors. For example, in an alternate embodiment shown inFIG. 9, holding tank 100 comprises four housing outlets 124, each beingupstream from a conduit 125, and four pairs 132 of screw conveyors 126.

Referring still to FIGS. 5-7, at least one of the screw conveyors 126,and preferably all of the screw conveyors 126, has a variable pitchalong its length. That is, the pitch of helical flight 130 is notconstant along the length L of at least one of the screw conveyors 126.

For example, in the embodiments shown, each screw conveyor has a firstend 158 proximal to its respective housing outlet 124 (i.e. the outlettowards which it conveys cellulosic feedstock), and a second end 156distal to its respective housing outlet 124 (shown in FIG. 7). The pitchof helical flight 130 at first end 158 is greater or wider than thepitch of helical flight 130 at second end 156. For example, the pitch atthe first end may be between about 14 inches and about 88 inches, andthe pitch at the second end may be between about 4 inches and about 8inches.

In the embodiments shown, the pitch of each helical flight 130 variescontinuously, and preferably at a constant rate, between the first end158 and the second end 156. That is, the pitch gradually becomes widertowards each discharge member outlet 124. In alternate embodiments, anabrupt transition between wider and narrower regions of flight mayoccur. For example, each screw conveyor may have a first regionextending from first end 158 towards a mid-point of screw conveyor 126,and a second region extending from second end 156 towards the midpoint.The first region may have a first range of pitch and the second regionhas a second range of pitch. For example, the first range of pitch maybe between about 14 inches and about 18 inches, and the second range ofpitch may be between about 4 inches and about 8 inches. In yet anotherembodiment, each screw conveyor may comprise an intermediate regionbetween the first region and the second region, and the intermediateregion may have a third range of pitch that is less than the first rangeof pitch and more than the second range of pitch. For example, the thirdrange of pitch may be between about 6 inches and about 10 inches.

Preferably, the screw conveyors 126 of each pair 132 have the same pitchat any location along their lengths. That is, the helical flight ofscrew conveyors 126 a and 126 b is essentially identical, and thehelical flight of screw conveyors 126 c and 126 d is essentiallyidentical.

Furthermore, the pitch of a first pair of screw conveyors is preferablya mirror image of the pitch of a second pair of screw conveyors, whichconvey the cellulosic feedstock in a direction opposite to the firstpair of screw conveyors. That is, the pitch of screw conveyors 126 a and126 b, which convey cellulosic material in direction A2, is a mirrorimage of the pitch of screw conveyors 126 c and 126 d, which conveycellulosic material in a direction A4.

Providing each screw conveyor with a variable pitch, and morespecifically with a narrower pitch distal to the housing outlet permitsmore equal amounts, and may allow for substantially equal amounts, ofcellulosic feedstock to be withdrawn from each portion of outlet 112.That is, material deposited in screw conveyor 126 at the distal end 156will be conveyed towards the respective outlet 124 for that screwconveyor. As that material is transported laterally, the pitch of thescrew increases permitting additional material to be deposited directlyin the screw conveyor from outlet 112. Further increases in the pitchwill permit additional portions of the material to fall into screwconveyor. The portion or portions of the screw conveyor closer to outlet124 (in the direction of transport) has a wider pitch such that it mayaccommodate material conveyed from the distal region, as well asmaterial deposited directly thereon from passage 104. Accordingly,feedstock is withdrawn from across all of outlet 112.

Referring to FIGS. 5 and 10, holding tank apparatus 100 preferablyfurther comprises a heating jacket 136 provided on at least a portion ofholding tank apparatus 100. Preferably, the at least one sidewall 102 isprovided with a heating jacket. For example, in the embodiment shown,heating jacket 136 surrounds all of each sidewall 102. Heating jacket136 may comprise a plurality of outer walls that are generally parallelto and spaced from sidewalls 102 so as to define an enclosure 142therebetween. A fluid may be passed through enclosure 142 from an inlet(not shown) to an outlet (not shown) so that a heated fluid is passedthrough enclosure 142. Heating jacket 136 may be of any constructionknown in the art. Accordingly, the cellulosic material may be heated toa predetermined temperature, or maintained at a predeterminedtemperature as it passes through holding tank apparatus 100.

Referring to FIG. 5, in a further preferred embodiment, housing 116 alsocomprises a second heating jacket 146 provided by housing 116. In theembodiment shown, heating jacket 146 is configured similarly to heatingjacket 136, and may comprise an outer wall 154 spaced outwardly fromsidewalls 120 and/or base 118 and is configured for passing a heatedfluid through an enclosure 150 defined between outer walls 154 andsidewalls 120 and/or base 118. Heating jacket 146 may be of anyconstruction known in the art.

In some embodiments, one or more temperature sensors may be provided inpassage 104. For example, a first thermocouple (not shown) may beprovided in the upper portion 106 of passage 104, to measure thetemperature of the cellulosic feedstock entering inlet 110, and a secondthermocouple (not shown) may be provided in the lower portion 108 ofpassage 104, to measure the temperature of the cellulosic feedstockexiting outlet 112. In some embodiments, one or more displays (notshown) may be coupled to the one or more temperature sensors, such thata user may view the measured temperatures, and optionally, adjust theamount of heat provided to holding tank 100 based on the measuredtemperatures. In further embodiments, the one or more sensors may becoupled to a processor, which may automatically adjust the amount ofheat provided to holding tank 100 based on the measured temperatures.

A method of treating a cellulosic feedstock that may be used for ethanolproduction will now be described. Although the method will be describedwith reference to holding tank apparatus 100, it will be appreciatedthat the method may be carried out using an alternate apparatus, andholding tank 100 may be operated according to an alternate method.

A suitable cellulosic feedstock is preferably first subjected tomoisture impregnation to raise the moisture content of the feedstock toa predetermined level prior to entry to the holding tank. Preferably,the moisture content of the feedstock upon entry to the holding tank isfrom about 30 wt % to about 60 wt %, preferably from about 45 wt % toabout 55 wt %. The cellulosic feedstock may be obtained from, forexample, a pre-treatment device such as impregnator 10, in whichmoisture is added to the cellulosic feedstock to raise the moisturecontent from, e.g., less than about 15% to between about 30% and about60 wt % upon entry to the holding tank. Preferably, the moisture contentis between about 45 wt % and about 55 wt % upon entry to the holdingtank.

The cellulosic feedstock is passed from the outlet of the impregnationchamber to the inlet of a holding tank. For example the cellulosicfeedstock may be passed from outlet 18 of impregnation chamber 10, intoinlet 110 of holding tank 100.

The cellulosic feedstock, with or without being subjected toimpregnation, is then preferably passed downwardly through the holdingtank. For example, referring to holding tank 100, inlet 110 is disposedat an elevation above outlet 112. Accordingly, the material may migratedownwardly from the inlet towards the outlet under the force of gravity.Furthermore, in embodiments wherein lower portion 108 has a greatercross sectional area than upper portion 106, the material will furthermigrate laterally as it migrates downwardly.

Preferably a generally constant residence time is maintained in theholding tank. That is, the holding tank is preferably operatedcontinuously at steady state conditions, such that all parts of eachportion or layer of feedstock added at inlet 110 pass downwardly tooutlet 112 at about the same rate. This result may be achieved bywithdrawing feedstock from all portions of the outlet 112. For example,the material may be removed from the outlet by operating one or morescrew conveyors, such as screw conveyors 126 described hereinabove, suchthat feedstock from all parts of the outlet 112 (e.g., all of the samehorizontal layer of feedstock in the outlet 112) are collectedconcurrently in the screw conveyor and transported to an outlet ordownstream passage. It will be appreciated that the amount of materialthat is withdrawn from each part of the holding tank outlet may bevaried by adjusting the pitch of the flight of the screw conveyor. Byenlarging the pitch at certain locations, the amount of feedstockwithdrawn at those locations may be increased.

It will be appreciated that, in use, there may be an initial start upphase, wherein material is not removed from the holding tank, and thetank is filled with cellulosic feedstock from impregnation chamber 12.

The residence time may be, for example, between about 10 and about 30minutes. An advantage of this method is that a generally uniformresidence time of the feedstock in the vessel may be achieved. Forexample, the variance of the residence time may be up to 5 minutes,preferably, less than 3 minutes and more preferably less than 2 minutes.

In one embodiment, the method comprises laterally conveying thecellulosic feedstock from the passage of the holding tank. Accordingly,once the feedstock reaches the exit (outlet) of the holding tank, thefeedstock is conveyed laterally to, e.g., one or more conduits in flowcommunication with a downstream process unit. For example, the holdingtank may comprise a discharge member, such as discharge member 116,adjacent lower end 108. One or more screw conveyors 126 of the dischargemember may convey the cellulosic feedstock laterally across outlet 112,as described hereinabove.

Alternately, or in addition, in other embodiments, the cellulosicmaterial is actively withdrawn from essentially the entirety of outlet112. The feedstock is therefore moved at least with the assistance ofmachinery out of the holding tank. For example, discharge member 116 maycomprise a plurality of screw conveyors extending across outlet 112,which, when rotated, engage the cellulosic material adjacent theentirety of the outlet 112, and convey it towards a discharge memberoutlet, as described hereinabove.

Alternately, or in addition, in other embodiments, generally equallyamounts of the cellulosic material are preferably withdrawn from eachportion of the outlet 112. In such an embodiment, a screw conveyorhaving different pitches (progressively wider pitches) may be used toconvey the cellulosic feedstock laterally across the outlet of theholding tank. For example, a screw conveyor 126 may have a helicalflighting having a first pitch adjacent a discharge member outlet, andsecond pitch narrower than the first pitch distal to the dischargemember outlet. Accordingly a generally equal amount of feedstock may bewithdrawn from the region adjacent a discharge member outlet, and from aregion distal to a discharge member outlet.

In some embodiments, a first portion of the cellulosic feedstock ispreferably withdrawn in a first lateral direction and a second portionof the cellulosic feedstock withdrawing a second portion of thecellulosic feedstock is preferably in a second lateral direction, whichis preferably opposite to the first direction. For example, the holdingtank may comprise a first pair of screw conveyors and a second pair ofscrew conveyors. Each screw conveyor may comprise a right-handed helicalflighting, and the first pair of screw conveyors may be rotated in afirst direction to convey the cellulosic feedstock in a first lateraldirection, and the second pair of screw conveyors may be rotated in asecond direction to convey the cellulosic feedstock in a second lateraldirection.

In some embodiments, the method further comprises maintaining atemperature in the holding tank between about 50° C. and about 75° C.For example, the holding tank may be optionally provided with a heatingjacket, such as heating jacket 136 and/or discharge member 116 may beoptionally provided with a heating jacket 146. The heating jacket mayserve to heat the walls of the holding tank and/or the discharge member,such that the material within the holding tank is maintained at orraised to a temperature between 50° C. and about 75° C.

It will be appreciated that certain features of the invention, whichare, for clarity, described in the context of separate embodiments orseparate aspects, may also be provided in combination in a singleembodiment. Conversely, various features of the invention, which are,for brevity, described in the context of a single embodiment or aspect,may also be provided separately or in any suitable sub-combination.

Although the invention has been described in conjunction with specificembodiments thereof if is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. In addition, citation or identification of anyreference in this application shall not be construed as an admissionthat such reference is available as prior art to the present invention.

We claim:
 1. A holding tank apparatus for preparing a cellulosicfeedstock, comprising: (a) at least one sidewall defining a volumehaving an upper portion and a lower portion; (b) at least one inletadjacent the upper portion; (c) at least a one outlet adjacent the lowerportion, at an elevation below the inlet; and, (d) a plurality of screwconveyors comprising a first pair of screw conveyors and a second pairof screw conveyors for conveying the cellulosic feedstock laterallyacross the outlet, the first and second pairs of screw conveyors havinga variable pitch along their respective lengths, wherein the first pairof screw conveyors convey cellulosic feedstock in a first direction andthe second pair of screw conveyors conveys cellulosic feedstock in anopposite direction.
 2. The apparatus of claim 1, wherein the first andsecond pairs of screw conveyors each have a first end and a second end,and the pitch at the first end of each pair of conveyors differs fromthe pitch at the respective second end.
 3. The apparatus of claim 2,wherein the first and second pairs of screw conveyors each comprise afirst region having a first range of pitch, and a second region having asecond range of pitch.
 4. The apparatus of claim 3, the first and secondpairs of screw conveyors each further comprising an intermediate regionbetween their respective first and second regions, the intermediateregion having a third range of pitch between the first range of pitchand the second range of pitch.
 5. The apparatus of claim 1, wherein thefirst and second pairs of screw conveyors each have a first end and asecond end, and the pitch decreases from each first end to therespective second end.
 6. The apparatus of claim 5, wherein the pitch ofthe first and second pairs of screw conveyors varies at a constant ratebetween each first end and the respective second end.
 7. The apparatusof claim 5, wherein the outlet defines a plurality of portions and thepitch of each of the first and second pairs of screw conveyors variessuch that approximate equal portions of feedstock are withdrawn fromeach portion of the outlet.
 8. The apparatus of claim 1, wherein thepitch of the first pair of screw conveyors at any location along theirlength is essentially the same and the pitch of the second pair of screwconveyors at any location along their length is essentially the same. 9.The apparatus of claim 1, wherein the pitch of the first pair of screwconveyors is a mirror image of the pitch of the second pair of screwconveyors.
 10. The apparatus of claim 1, wherein the plurality of screwconveyors extends across all of the outlet.
 11. The apparatus of claim1, further comprising a heating jacket provided on at least a portion ofthe apparatus.
 12. The apparatus of claim 1, wherein the lower portionhas a greater cross sectional area then the upper portion.
 13. Theapparatus of claim 1, wherein the at least one sidewall comprises afirst sidewall and a second sidewall opposed to the first sidewall, thefirst and second sidewalls diverging relative to each other from theupper portion to the lower portion.
 14. The apparatus of claim 13,wherein the at least one sidewall comprises a third sidewall and afourth sidewall opposed to the third sidewall, the third and fourthsidewalls extending between the first and second sidewalls, the thirdand fourth sidewalls diverging relative to each other from the upperportion to the lower portion.
 15. A holding tank apparatus for preparinga cellulosic feedstock, comprising: (a) at least one sidewall defining avolume having an upper portion and a lower portion; (b) at least oneinlet adjacent the upper portion; (c) at least a one outlet adjacent thelower portion, at an elevation below the inlet; and, (d) a plurality ofscrew conveyors comprising a first pair of screw conveyors and a secondpair of screw conveyors for conveying the cellulosic feedstock laterallyacross the outlet, the first and second pairs of screw conveyors havinga variable pitch, at least one of the plurality of screw conveyorsconveys cellulosic feedstock in a first direction and at least one ofthe plurality of screw conveyors conveys cellulosic feedstock in asecond direction, wherein the first direction and the second directionare substantially opposite, and wherein the first pair of screwconveyors is rotatable in a first direction of rotation, and the secondpair of screw conveyors is rotatable in a direction of rotation oppositethe first direction of rotation.
 16. The apparatus of claim 15, whereinthe first and second pairs of screw conveyors each have a first end anda second end, and the pitch at the first end of each pair of conveyorsdiffers from the pitch at the respective second end.
 17. The apparatusof claim 16, wherein the first and second pairs of screw conveyors eachcomprise a first region having a first range of pitch, and a secondregion having a second range of pitch.
 18. The apparatus of claim 17,the first and second pairs of screw conveyors each further comprising anintermediate region between their respective first and second regions,the intermediate region having a third range of pitch between the firstrange of pitch and the second range of pitch.
 19. The apparatus of claim15, wherein the first and second pairs of screw conveyors each have afirst end and a second end, and the pitch decreases from each first endto the respective second end.
 20. The apparatus of claim 19, wherein thepitch of the first and second pairs of screw conveyors varies at aconstant rate between each first end and the respective second end. 21.The apparatus of claim 19, wherein the outlet defines a plurality ofportions and the pitch of each of the first and second pairs of screwconveyors varies such that approximate equal portions of feedstock arewithdrawn from each portion of the outlet.
 22. The apparatus of claim15, wherein the pitch of the first pair of screw conveyors at anylocation along their length is essentially the same and the pitch of thesecond pair of screw conveyors at any location along their length isessentially the same.
 23. The apparatus of claim 15, wherein the pitchof the first pair of screw conveyors is a mirror image of the pitch ofthe second pair of screw conveyors.
 24. The apparatus of claim 15,wherein the plurality of screw conveyors extends across all of theoutlet.
 25. The apparatus of claim 15, further comprising a heatingjacket provided on at least a portion of the apparatus.
 26. Theapparatus of claim 15, wherein the lower portion has a greater crosssectional area then the upper portion.
 27. The apparatus of claim 15,wherein the at least one sidewall comprises a first sidewall and asecond sidewall opposed to the first sidewall, the first and secondsidewalls diverging relative to each other from the upper portion to thelower portion.
 28. The apparatus of claim 27, wherein the at least onesidewall comprises a third sidewall and a fourth sidewall opposed to thethird sidewall, the third and fourth sidewalls extending between thefirst and second sidewalls, the third and fourth sidewalls divergingrelative to each other from the upper portion to the lower portion.